And I think there's another issue you have to consider whenever you are going to house more than one tortoise in an enclosure, no matter what species and genders you are mixing.
There is always a chance that the two (or more) animals will prove to be incompatible. Even the mellowest females may decide they don't wish to share space.
And then what do you do?
I'd say you either have to be willing to
1) Divide the current enclosure, or expand it, or create a new one so the animals can continue to live with you but not be exposed to each other. Of course this means you have to have more hideboxes, heat lamps, water bowls, etc. and maybe losing more space in your house and yard to house animals.
Or
2) Get rid of the new animal by selling or adopting it out.
So adding an animal requires more than considering the possibilities of disease transmission. You have to know what you are going to do if the animals just can't get along.
If you can't afford to re-arrange the housing situation, then maybe you shouldn't add an animal.